title: Electron paramagnetic resonance and electron nuclear double resonance studies of X-irradiated crystals of cytosine hydrochloride. Part I: free radical formation at 10 K after high radiation doses.
abstract: Anhydrous single crystals of cytosine hydrochloride (protonated at N3) have been X-irradiated at 10 K and studied using K-band EPR, ENDOR and FSE spectroscopy. At least seven radicals were present at 10 K after X irradiation with a dose of about 150 kGy. Two different protonation states of the one-electron reduced cytosine cation were observed: an amino-protonated species (R1) and the pristine one-electron reduced species (R2) with zero net charge. Apparently three deprotonated versions of the one-electron oxidized cytosine cation were formed: the amino-deprotonated cation (R3), an N3-deprotonated cation (R4) and an N1-deprotonated cation (R5). Finally, two products formed by net hydrogen addition to the cytosine base were observed: a C5 hydrogen-addition radical (R6) and a C6 hydrogen-addition radical (R7). The crystalline lattice of cytosine hydrochloride is characterized in part by a cytosine base initially protonated at the N3-position, thus forming a cytosine base cation, and in part by an extended network of hydrogen bonding involving the chlorine anions. Proton transfer properties of pristine one-electron oxidation and reduction base products in this lattice are discussed and are suggested as explanations of the unusual multitude of positions for deprotonation of the one-electron oxidized species as well as for the two protonation states of the reduction product observed. The magnetic parameters for the amino-protonated species R1 agree well with those extracted from previous studies of cytosine derivatives in frozen solutions and in various glasses.
authors.affiliation: Department of Physics, University of Oslo, Norway.
authors.forename: E O
authors.initials: EO
authors.lastname: Hole
authors.forename: W H
authors.initials: WH
authors.lastname: Nelson
authors.forename: E
authors.initials: E
authors.lastname: Sagstuen
authors.forename: D M
authors.initials: DM
authors.lastname: Close
mesh_terms: D003461:Crystallography; D003596:Cytosine / Q000737:chemistry / Q000528:radiation effects*; D003903:Deuterium; D004307:Dose-Response Relationship, Radiation; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D010084:Oxidation-Reduction; D014965:X-Rays
chemical_list: D005609:Free Radicals; D003596:Cytosine; D003903:Deuterium